The instant invention relates to an adjustable viscous shear coupling and to a process for distributing the driving power between the axles of an all-wheel drive vehicle. The viscous shear coupling is adjusted by shifting a regulating piston of the viscous shear coupling by means of a control circuit containing a control valve on the basis of signals determined as a function of driving conditions.
A controllable viscous shear coupling system to drive a motor vehicle with two driven axles is known for example from the present assignee's EP-PS 332 608 (U.S. 4,942,951), wherein the coupling is located in the drive train towards the rear axle and is combined with a planetary gear system. Control is effected by means of hydraulic shifting of an adjusting piston in the coupling which changes the distances between disks. The design of the control system and its manner of operation for controlling movement of the piston is not described in that document.
DE-OS 36 26 025 discloses a control arrangement for a multiple disk friction clutch for the hydraulically controlled hook-up of the front wheel drive of a motor vehicle. In this control arrangement, a signal for the control of a valve is derived from signals determined as a function of driving conditions, said signal determining the contact pressure in the multiple disk clutch. The control device merely compares the difference between rotational speeds of the axles with a comparison value determined as a function of speed and steering angle and calculates from this a desired driving torque for the hooked-up front axle through multiplication by various coefficients, depending on whether the difference in axle speeds is above or below the comparison value. This desired value is then set by changing the pressure in the hydraulic control circuit.
Aside from the fact that this latter control arrangement involves a friction clutch with a different action and adjustment behavior from a viscous shear coupling, essential factors such as driving dynamics (e.g. engine torque) and driving conditions are not taken into consideration. As a result, the control reacts only to the slip so to say trailing behind it, and is therefore relatively sluggish so that the possibilities of variable driving power distribution are not utilized with respect to traction nor with respect to driving safety.
A similar drive arrangement with multiple-disk clutch is also known from DE-PS 35 05 455. Although in this device the control does take several factors of driving dynamics into consideration, among them tire defects, the clutch can only be opened completely or closed completely.
It is therefore the object of the instant invention to provide an adjustable viscous shear coupling and a regulating process for it making it possible to achieve driving power distribution control that would be as precise as possible, would reflect the physical conditions, and would be as rapid as possible and to some extent even anticipatory, while at the same time reflecting as precisely as possible all essential factors of driving dynamics.